Semiconductor processing tools often use radical sources to distribute radicalized process gas across a semiconductor wafer during processing, e.g., during atomic layer deposition (ALD) processing. Such radical sources may include a faceplate that faces the wafer during processing, and a number of gas distribution holes may be distributed across the faceplate to facilitate radicalized gas delivery to the wafer from within the radical source.
During some semiconductor manufacturing processes, e.g., plasma-enhanced atomic layer deposition (PEALD), semiconductor fabrication process gases may be converted into a plasma to produce radicals used in various process steps. Such plasma-enhanced processes may provide advantages over, for example, thermal atomic layer deposition since such processes may be performed with lower process temperatures and greater flexibility in process chemistry, and may provide denser deposition films. Plasma conversion, however, may also be damaging to the wafer, e.g., by oxidizing the underlying silicon of the wafer or an ultra-low K dielectric used in the process. To reduce such damage potential, such plasmas may be located so as to be “remote” from the wafer; such processes are commonly referred to as remote plasma atomic layer deposition (RPALD) processes. For example, some radical sources may have an internal volume within which the plasma may be generated. This internal volume may be separated from the wafer by the radical source faceplate (making the plasma “remote” from the wafer), somewhat shielding the wafer from possible damage arising from plasma conversion. The gas distribution holes in the faceplate may allow radicals produced by the remotely-generated plasma to flow out of the radical source and onto the wafer.
FIG. 1 depicts a conceptual, high-level flow chart for an RPALD technique. The process may begin in block 102, where a wafer may be introduced into an RPALD reactor for processing. In block 104, a precursor may be flowed into the reactor and across the wafer. The precursor then experiences a largely self-limiting reaction with the wafer that forms a deposition layer on the wafer of a highly-uniform conformality and having low thickness. ALD films are of a uniform thickness on every surface of a wafer—the tops of islands as well as the sides and bottoms of trenches—providing 100% step coverage, or a nearly perfect conformal coating, regardless of feature size or aspect ratio. Given the largely self-limiting nature of the reaction, the thickness of the deposited layer is much more insensitive to process parameter variations than other semiconductor processes, such as plasma enhanced chemical vapor deposition (PECVD). The thickness of an RPALD-deposited layer may, for example, be largely determined by parameters such as precursor selection, wafer material selection, and process temperature. After the precursor-wafer reaction has been allowed to occur, the remaining, unreacted precursor may be purged from the reactor in block 106. In block 108, radicals may be flowed into the reactor and across the wafer from a remote plasma source. The radicals may then react with the deposited precursor. This reaction may alter the precursor film on the wafer and make the precursor film capable of reacting with further precursor gas that is flowed across the wafer, allowing for a further layer of precursor to be deposited on the wafer. After the radical/precursor film reaction has been allowed to occur, the remaining radicals in the reaction chamber may be purged in block 110. In block 112, a decision may be made as to whether the deposited film is of the desired thickness (or, alternatively, whether sufficient reaction cycles have been performed). If additional thickness is desired, the process may return to block 104. If the thickness is at the desired level, the process may end in block 114.
The purge cycles are needed to mitigate or eliminate the possibility that the precursor gas will mix with the radicalized gas that is flowed into the reactor, and vice-versa. Such precursor/radicalized gas mixing can lead to precursor/radical reactions that, in effect, transform an ALD process into a chemical vapor deposition (CVD) process. Since one of the benefits of ALD, PEALD, and RPALD is that such processes allow for thin film depositions of much higher conformality than CVD processes, such precursor/radical mixing is highly undesirable. Another undesirable side effect of such mixing is that some ALD chemistries may, when mixed, form particulates that may interfere with ALD processing, e.g., by creating electrical shorts or other problems. Accordingly, each precursor and radical flow into the reactor is separated by a flow of purge gas into the reactor.
While ALD-type processes provide superior film uniformity as compared with CVD films, ALD-type processes are generally slower than CVD processes since ALD requires that a film be built up by many sequential reaction cycles (a single reaction cycle may, for example, correspond to blocks 104 through 110 of FIG. 1) rather than as a single layer deposition during one CVD reaction process.